IP Multimedia Subsystem (IMS) is a subsystem proposed by 3rd Generation Partnership Project (3GPP), which is capable of supporting IP Multimedia service. The significant features of IMS are that Session Initiation Protocol (SIP) is adopted, and communication thereof has nothing to do with access mode, multiple media services can be provided, and there is a layered-manner design between controlling function and bearing function, call and session, application and service, service and network, and there is a mergence between mobile network and Internet service. IMS is in conformity to the trend of mergence development of telecommunication network.
In IMS R6, a shared public user identity (PUID) is introduced to allow one PUID to be shared by a plurality of user equipments (UE), wherein, the PUID shared by the plurality of UEs is referred to as shared PUID. The utilization of the shared PUID is fairly beneficial to the cooperative work modes, e.g. service hotline, emergency call 110, etc. Meanwhile, it can also be shared among different UEs of the same user, e.g. home fixed phone, office fixed phone and mobile phone, etc.
However, the use of the shared PUID also brings some problems that the called party cannot get to know which UE is the real calling party if the call is initiated by a calling party with a shared PUID, because the shared PUID is being used by multiple users. For instance, consider the scenario that user A received a call from user B with a shared PUID adopted for some issues. After the call is terminated, the user A may not call back to the user B for more detailed discussion because he doesn't know from which UE the call is made.
Moreover; in the case that a plurality of UEs share a PUID, the plurality of UEs cannot communicate with each other directly.
Although the contact address can be used to identify a plurality of UEs which share a PUID, it is not bound to a UE permanently and may be changed in the next registration.
In the prior art, for addressing the above problems, there is proposed a mechanism of SIP Globally Routable User Agent URIs, but still with the following drawbacks for GRUU:
Since it's a combination with UE, GRUU is bonded to the UE instead of the user; therefore, once the user switches to register on another UE, the other users could not find him from the GRUU he saved previously.
Additionally, the use of GRUU depends on how much a UE supports SIP. In IMS network, as SIP is not supported by those legacy UEs, GRUU mechanism can not be used.